Methods and electrolytes for electrodeposition of smooth films

ABSTRACT

Electrodeposition involving an electrolyte having a surface-smoothing additive can result in self-healing, instead of self-amplification, of initial protuberant tips that give rise to roughness and/or dendrite formation on the substrate and/or film surface. For electrodeposition of a first conductive material (C1) on a substrate from one or more reactants in an electrolyte solution, the electrolyte solution is characterized by a surface-smoothing additive containing cations of a second conductive material (C2), wherein cations of C2 have an effective electrochemical reduction potential in the solution lower than that of the reactants.

CROSS-REFERENCE TO RELATED APPLICATIONS

This invention claims priority from, and is a continuation in part of,currently pending U.S. patent application Ser. No. 13/367,508, filedFeb. 7, 2012, incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under ContractDE-AC0576RL01830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

BACKGROUND

Electrodeposition is widely used to coat a functional material having adesired property onto a surface that otherwise lacks that property.During electrodeposition, electrically charged reactants in anelectrolyte solution diffuse, or are moved by an electric field, tocover the surface of an electrode. For example, the electrical currentcan reduce reactant cations to yield a deposit on an anode. Or, anionsof reactants in the electrolyte solution can diffuse, or be moved by theelectric field, to cover the surface of a cathode, where the reactantanions are oxidized to form a deposit on the electrode.

Electrodeposition has been successfully utilized in the fields ofabrasion and wear resistance, corrosion protection, lubricity, aestheticqualities, etc. It also occurs in the operation of certain energystorage devices. For example, in the charge process of a metal batteryor metal-ion battery, metal ions in the electrolyte move from thecathode and are deposited on the anode. Some organic compounds withunsaturated carbon-carbon double or triple bonds are used as additivesin non-aqueous electrolytes and are electrochemically reduced anddeposited at the anode surface or oxidized and deposited at the cathodesurface to form solid electrolyte interphase layers as protection filmson both anode and cathode of lithium batteries. Some other organiccompounds with conjugated bonds in the molecules are electrochemicallyoxidized and deposited at the cathode surface to form electricconductive polymers as organic cathode materials for energy storagedevices.

In most instances, the ideal is a smooth electrodeposited coating. Forexample, a smoothly plated film can enhance the lifetime of a film usedfor decoration, wear resistance, corrosion protection, and lubrication.A smoothly plated film is also required for energy storage devices,especially for secondary devices. Rough films and/or dendrites generatedon electrode surfaces during the charge/discharge processes of theseenergy storage devices can lead to the dangerous situations,short-circuits, reduced capacities, and/or shortened lifetimes.

Roughness and/or dendrites can be caused by several reasons, includingthe uneven distribution of electric current density across the surfaceof the electrodeposition substrate (e.g., anode) and the unevenreactivity of electrodeposited material and/or substrate to electrolytesolvents, reactants, and salts. These effects can be compounded in theparticular case of repeated charging-discharging cycles in energystorage devices. Therefore, a need for improved electrolytes and methodsfor electrodeposition are needed to enhance the smoothness of theresultant film.

SUMMARY

This document describes methods and electrolytes for electrodepositionthat result in self-healing, instead of self-amplification, of initialprotuberant tips, which are unavoidable during electrodeposition andwhich give rise to roughness and/or dendrite formation. Forelectrodeposition of a first conductive material (C1) on a substratefrom one or more reactants in an electrolyte solution, embodiments ofthe electrolyte solution described herein are characterized by asoluble, surface-smoothing additive comprising cations of a secondconductive material (C2), wherein cations of C2 have an effectiveelectrochemical reduction potential (ERP) in the solution lower thanthat of the reactants.

As used herein, cations, in the context of C1, C2, and/or reactants,refer to atoms or molecules having a net positive electrical charge. Inbut one example, the total number of electrons in the atom or moleculecan be less than the total number of protons, giving the atom ormolecule a net positive electrical charge. The cations are notnecessarily cations of metals, but can also be non-metallic cations. Atleast one example of a non-metallic cation is ammonium. Cations are notlimited to the +1 oxidation state in any particular instance. In somedescriptions herein, a cation can be generally represented as X⁺, whichrefers generally to any oxidation state, not just +1.

In another example, the reactants might not technically be cations butare positively charged species such as conductive monomers/polymers.During the electrodeposition of a metal cation, the cation gets theelectron at the anode and is reduced to metal. When forming a conductivepolymer via electrodeposition, it is the conjugated monomer, which canbe neutral but with double or triple bonds, that gets the electrons. Theconjugated monomer re-arranges the double or triple bonds among the samemolecular structure and forms new bonds among different molecules. Theformed polymer is either neutral or positively charged when protons areincorporated onto the polymer moiety.

In one embodiment, C1 is a metallic material and the reactants comprisecations of C1. Examples of suitable metallic materials include, but arenot limited to, elemental metals or alloys containing Li, Na, K, Rb, Cs,Be, Mg, Ca, Sr, Ba, Al, Ga, In, Tl, Ge, Sn, Pb, As, Sb, Bi, Se, Te, Bi,Po, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh,Pd, Ag, Cd, W, Pt, Au, and/or Hg. Preferably, C1 is an elemental metalmaterial comprising Li, Zn, Na, Mg, Al, Sn, Ti, Fe, Ni, Cu, Zn, Ag, Pt,or Au.

Alternatively, C1 can comprise an electronic conductive polymer. In suchinstances, the reactants can comprise monomers of the polymer. Themonomers can be conjugated monomers that are reduced at the anode duringdeposition. Examples of polymers can include, but are not limited to,polyanaline, polypyrrole, polythiophene,poly(3,4-ethylenedioxythiophene). Monomers of these polymers caninclude, but are not limited to, analine, pyrrole, thiophen,3,4-ethylenedioxythiophene, respectively.

In another embodiment, the cations of C2 are metal cations. Examples ofmetals for cations of C2 include, but are not limited to, Li, Cs, Rb, K,Ba, La, Sr, Ca, Ra, Zr, Te, B, Bi, Ta, Ga, Eu, S, Se, Nb, Na, Mg, Cu,Al, Fe, Zn, Ni, Ti, Sn, Sb, Mn, V, Ta, Cr, Au, Ge, Co, As, Ag, Mo, Si,W, Ru, I, Fc, Br, Re, Bi, Pt, and/or Pd. In preferred embodiments,cations of C2 are cations of Cs, Rb, K, Ba, Sr, Ca, Li.

A cation of C2 might have a standard reduction potential that is greaterthan that of the reactants. In such instances, some embodiments of theelectrolytes have an activity of C2 cations such that the effective ERPof the C2 cations is lower than that of the reactants (C1). Becauseactivity is directly proportional to the concentration and activitycoefficient, which depend on the mobility and solvation of the cation inthe given electrolyte, a lower activity can be a result of lowconcentration, low activity coefficient of the cations, or both sincethe activity is the product of the activity coefficient andconcentration. The relationship between effective ERP and activity isdescribed in part by the Nernst equation and is explained in furtherdetail elsewhere herein. In a particular embodiment, the concentrationof C2 cations is less than, or equal to, 30% of that of the reactants.In another, the concentration of C2 cations is less than, or equal to,10% of that of the reactants. In yet another, the concentration of C2cations is less than, or equal to, 5% of that of the reactants

The surface-smoothing additive can comprise an anion that includes, butis not limited to, PF₆ ⁻, AsF₆ ⁻, BF₄ ⁻, N(SO₂CF₃)₂ ⁻, N(SO₂F)₂ ⁻,CF₃SO₃ ⁻, ClO₄ ⁻, I⁻, Cl⁻, OH⁻, NO₃ ⁻, SO₄ ²⁻, and combinations thereof.Preferably, the anion comprises PF₆ ⁻.

In one embodiment, the substrate is an electrode. For example, thesubstrate on which electrodeposition occurs can be an electrode in anenergy storage device. In particular instances, the electrode cancomprise lithium, carbon, magnesium, and/or sodium. As used herein,electrode is not restricted to a complete structure having both anactive material and a current collector. For example, an electrode caninitially encompass a current collector on which active material iseventually deposited to form an anode. Alternatively, an electrode canstart out as an active material pasted on a current collector. Afterinitial cycling, the active material can be driven into the currentcollector to yield what is traditionally referred to as an electrode.

Preferably, the cations of C2 are not chemically or electrochemicallyreactive with respect to C1 or the reactants. Accordingly, thesurface-smoothing additive is not necessarily consumed duringelectrodeposition.

The electrolyte also comprises a solvent. Examples of solvents caninclude, but are not limited to, water or a non-aqueous polar organicsubstance that dissolves the solutes at room temperature. Blends of morethan one solvent can be used. When water or a protic organic substanceis used as the solvent, C1 is not a metal that reacts with water or theprotic organic substance. Generally, organic solvents can include, butare not limited to, alcohols, ethers, aldehydes, ketones, carbonates,carboxylates, lactones, phosphates, nitriles, sulfones, amides, five orsix member heterocyclic ring compounds, and organic compounds having atleast one C₁-C₄ group connected through an oxygen atom to a carbon.Lactones may be methylated, ethylated and/or propylated. Other organicsolvents can include methanol, ethanol, acetone, sulfolane, dimethylsulfone, ethyl methyl sulfone, ethylene carbonate, propylene carbonate,dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, tetrahydrofuran, 2-methyl tetrahydrofuran,1,3-dioxolane, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane,1,2-dibutoxyethane, acetonitrile, dimethylformamide, methyl formate,ethyl formate, propyl formate, butyl formate, methyl acetate, ethylacetate, propyl acetate, butyl acetate, methyl propionate, ethylpropionate, propyl propionate, butyl propionate, methyl butyrate, ethylbutyrate, propyl butyrate, butyl butyrate, gamma-butyrolactone,2-methyl-gamma-butyrolactone, 3-methyl-gamma-butyrolactone,4-methyl-gamma-butyrolactone, delta-valerolactone, trimethyl phosphate,triethyl phosphate, tris(2,2,2-trifluoroethyl)phosphate, tripropylphosphate, triisopropyl phosphate, tributyl phosphate, trihexylphosphate, triphenyl phosphate, and combinations thereof. Still othernon-aqueous solvents can be used so long as they are capable ofdissolving the solute salts.

Methods for improving surface smoothness during electrodeposition of C1on a substrate surface can comprise providing an electrolyte solutioncomprising reactants from which C1 is deposited and a soluble,surface-smoothing additive comprising cations of a second conductivematerial (C2) and applying an electrical potential thereby reducing thereactants and forming C1 on the substrate surface. The cations of C2have an effective electrochemical reduction potential in the solutionlower than that of the reactants. In preferred embodiments, the methodsfurther comprise accumulating cations of C2 at protrusions on thesubstrate surface, thereby forming an electrostatically shielded regionnear each protrusion. The electrostatically shielded region cantemporarily repel reactants, thus reducing the local effective currentdensity and slowing deposition at the protrusion while enhancingdeposition in regions away from the protrusions. In this way, the growthand/or amplification of the protrusions are suppressed and the surfaceheals to yield a relatively smoother surface.

In one embodiment, the method is applied to electrodeposition of lithiumon a substrate surface. Lithium is an effective example because Li⁺ ionshave the lowest standard ERP among metals (at a concentration of 1mol/L, a temperature of 298.15 K (25° C.), and a partial pressure of101.325 kPa (absolute) (1 atm, 1.01325 bar) for each gaseous reagent).C2 cations, which have standard EPR values that are greater than lithiumcations can have activity-dependent effective ERP values that are lowerthan those of the lithium cations.

According to such embodiments, the method comprises providing anelectrolyte solution comprising lithium cations and a soluble,surface-smoothing additive comprising cations of a second conductivematerial (C2) selected from the group consisting of cesium, rubidium,potassium, strontium, barium, calcium, and combinations thereof. Thecations of C2 have a concentration and activity coefficient in solutionsuch that the effective electrochemical reduction potential of thecations of C2 is lower than that of the lithium cations. The methodfurther comprises applying an electrical potential, thereby reducing thelithium cations and forming lithium on the substrate surface. The methodfurther comprises accumulating cations of C2 at protrusions on thesubstrate surface, thereby forming an electrostatically shielded regionnear each protrusion and temporarily repelling the lithium cations fromthe electrostatically shielded regions. In some instances, theelectrostatically shielded region has a higher impedance to retard thefurther deposition of lithium cations.

In particular embodiments, the concentration of C2 cations is less than,or equal to 30% of that of the lithium cations. In others, the C2 cationconcentration is less than, or equal to, 5% of that of the lithiumcations. Preferably, the surface-smoothing additive comprises an anioncomprising PF₆ ⁻ anion. The substrate can be a battery anode thatcomprises lithium or that comprises carbon.

The purpose of the foregoing abstract is to enable the United StatesPatent and Trademark Office and the public generally, especially thescientists, engineers, and practitioners in the art who are not familiarwith patent or legal terms or phraseology, to determine quickly from acursory inspection the nature and essence of the technical disclosure ofthe application. The abstract is neither intended to define theinvention of the application, which is measured by the claims, nor is itintended to be limiting as to the scope of the invention in any way.

Various advantages and novel features of the present invention aredescribed herein and will become further readily apparent to thoseskilled in this art from the following detailed description. In thepreceding and following descriptions, the various embodiments, includingthe preferred embodiments, have been shown and described. Includedherein is a description of the best mode contemplated for carrying outthe invention. As will be realized, the invention is capable ofmodification in various respects without departing from the invention.Accordingly, the drawings and description of the preferred embodimentsset forth hereafter are to be regarded as illustrative in nature, andnot as restrictive.

DESCRIPTION OF DRAWINGS

Embodiments of the invention are described below with reference to thefollowing accompanying drawings.

FIGS. 1A-1F are illustrations depicting an embodiment ofelectrodeposition using an electrolyte having a surface-smoothingadditive.

FIGS. 2A-2D include SEM micrographs of Li films deposited in anelectrolyte with or without a surface-smoothing additive according toembodiments of the present invention; (a) No additive; (b) 0.05 M RbPF₆;(c) 0.05 M CsPF₆; (d) 0.15 M KPF₆.

FIGS. 3A-3B include SEM micrographs of pre-formed dendritic Li filmdeposited in a control electrolyte for 1 hour and the same film afteranother 14 hours' Li deposition in the electrolyte with additive (0.05MCsPF₆), respectively.

FIGS. 4A-4F include SEM micrographs of Li electrodes after repeateddeposition/stripping cycles in the control electrolytes (a, b, and c)and with Cs⁺-salt additive (d, e and f).

FIGS. 5A-5B include SEM micrographs of Li electrodes after 100 cycles incoin cells of Li|Li₄Ti₅O₁₂ containing electrolytes without (a) and with(b) 0.05 M Cs⁺ additive.

FIGS. 6A-6F include optical and SEM micrographs of hard carbonelectrodes after charging to 300% of the regular capacity in the controlelectrolyte (a, c, e) and in an electrolyte with 0.05 M CsPF₆ additiveadded in the control electrolyte (b, d, f).

DETAILED DESCRIPTION

The following description includes the preferred best mode of oneembodiment of the present invention. It will be clear from thisdescription of the invention that the invention is not limited to theseillustrated embodiments but that the invention also includes a varietyof modifications and embodiments thereto. Therefore the presentdescription should be seen as illustrative and not limiting. While theinvention is susceptible of various modifications and alternativeconstructions, it should be understood, that there is no intention tolimit the invention to the specific form disclosed, but, on thecontrary, the invention is to cover all modifications, alternativeconstructions, and equivalents falling within the spirit and scope ofthe invention as defined in the claims.

FIGS. 1-6 show a variety of embodiments and aspects of the presentinvention. Referring first to FIG. 1, a series of illustrations depictan embodiment of electrodeposition using an electrolyte 104 having asurface-smoothing additive. The additive comprises cations of C2 102,which have an effective ERP lower than that of the reactants 103. FIG. 1illustrates how an electrostatically shielded region 106 can developresulting in the self-healing of the unavoidable occurrence of surfaceprotrusions 105 that would normally form. During the initial stage ofdeposition, both the reactants and the cations of C2 are adsorbed on thesubstrate surface 100 (FIG. 1A) under an applied voltage (E_(a)) 101slightly less than the reduction potential of the reactant (E_(r)) butlarger than the additive reduction potential (E_(C2/C2′)), that is,E_(r)>E_(a)>E_(C2/C2′)). Reactants will be deposited to form C1 on thesubstrate and will unavoidably form some protuberance tips due tovarious fluctuations in the system (FIG. 1B). A sharp edge or protrusionon the electrode exhibits a stronger electrical field, which willattract more positively charged cations (including both C1 and C2).Therefore, more cations of C1 will be preferentially deposited aroundthe tips rather than on other smooth regions. In conventionalelectrodeposition, amplification of this behavior will form the surfaceroughness and/or dendrites. However, according to embodiments of thepresent invention, the adsorbed additive cations (C2⁺) have an effectiveERP lower than E_(a) (FIG. 1C) and will not be deposited (i.e.,electrochemically or chemically consumed, reacted, and/or permanentlybound) on the tip. Instead, they will be temporarily electrostaticallyattracted to and accumulated in the vicinity of the tip to form anelectrostatic shield (FIG. 1D). This positively charged shield willrepel incoming reactants (e.g., like-charged species) at the protrudedregion and force them to be deposited in non-protrusion regions. The neteffect is that reactants will be preferentially deposited in thesmoother regions of the substrate (FIG. 1E) resulting in a smootheroverall deposition surface (FIG. 1F). This process is persistentlyoccurring and/or repeating during electrodeposition. The self-healingmechanism described herein resulting from embodiments of the presentinvention appears to disrupt the conventional roughness and/or dendriteamplification mechanism and leads to the deposition of a smooth film ofC1 on the substrate.

The additive cation (C2⁺) exhibits an effective ERP, E_(Red), less thanthat of the cations (C1⁺) of the reactants. In some instances, thestandard ERP of the C2 cation will be less than that of the reactants.Surface-smoothing additives comprising such C2 species can be utilizedwith appropriate reactants with few limitations on concentration andactivity coefficient. However, in some instances, the C2 cation willhave a standard ERP that is greater than that of the reactants. Theconcentration and activity coefficient of the C2 cations can becontrolled such that the effective ERP of the C2 cations is lower thanthat of the reactant cations. For example, if the reactant is a Li⁺ ion,which has the lowest standard ERP among metals, then the concentrationand activity coefficient of C2 cations can be controlled such that theeffective ERP is lower than that of the lithium cations.

According to the Nernst equation:

$\begin{matrix}{E_{Red} = {E_{Red}^{\phi} - {\frac{R\; T}{z\; F}\ln\frac{\alpha_{Red}}{\alpha_{Ox}}}}} & (1)\end{matrix}$where R is the universal gas constant (=8.314 472 J K⁻¹ mol⁻¹), T is theabsolute temperature (assume T=25° C. in this work), α is the activityfor the relevant species (α_(Red) is for the reductant and α_(Ox) is forthe oxidant). α_(x)=γ_(x)c_(x), where γ_(x) and c_(x) are the activitycoefficient and the concentration of species x. F is the Faradayconstant (9.648 533 99×10⁴ C mol⁻¹), z is the number of moles ofelectrons transferred. Although Li⁺ ion has the lowest standardreduction potential, E_(Red) (Li⁺), among all the metals when measuredat a standard conditions (1 mol/L), a cation (M⁺) may have an effectivereduction potential lower than those of lithium ion (Li⁺) if M⁺ has anactivity α_(x) much lower than that of Li⁺. In the case of lowconcentration when the activity coefficient is unity, α can besimplified as concentration c_(x), then Eq. (1) can be simplified as:

$\begin{matrix}{E_{Red} = {E_{Red}^{\phi} - {\frac{0.05916\mspace{11mu} V}{z}\log_{10}\frac{1}{c_{Ox}}}}} & (2)\end{matrix}$

Table 1 shows several the reduction potentials for several cations (vs.standard hydrogen electrode (SHE)) at various concentrations assumingthat their activity coefficients, γ_(x), equal one. When theconcentration of Cs⁺, Rb⁺, and K⁺ is 0.01M in an electrolyte, theireffective ERPs are −3.144 V, −3.098 V and −3.049 V, respectively, whichare less than those of Li⁺ at 1M concentration (−3.040 V). As a result,in a mixed electrolyte where the additive (Cs⁺, Rb⁺, and K⁺)concentration is much less than Li⁺ concentration, these additives willnot be deposited at the lithium deposition potential. In addition to alow concentration c_(x), a very low activity coefficient γ_(x) (which isstrongly affected by the solvation and mobility of the cations in thegiven solvent and lithium salt) may also reduce the activity of cationsand lead to an effective reduction potential lower than that of thelithium ion (Li⁺) as discussed below.

TABLE 1 The effective reduction potential of selected cations vs. SHELi⁺ Cs⁺ Rb⁺ K⁺ Stand reduction potential −3.040 V −3.026 V −2.980 V−2.931 V (1M) Effective reduction potential — −3.103 V  −3.06 V  −3.01 Vat 0.05M* Effective reduction potential — −3.144 V −3.098 V −3.049 V at0.01M* *Assume the activity coefficient γ_(x) of species x equals 1.Surface Smoothing Exhibited in Electrodeposition of Lithium

Embodiments of the present invention are illustrated well in theelectrodeposition of lithium, since lithium ions have the loweststandard ERP among metals. However, the present invention is not limitedto lithium but is defined by the claims.

The effect of several C2 cations has been examined for use insurface-smoothing additives in the electrodeposition of lithium. Thecations all have standard ERP values, E_(Red) ^(φ), that are close tothat of Li⁺ ions. The electrolyte comprised 1M LiPF₆ in propylenecarbonate. Electrolyte solutions with surface-smoothing additivescomprising 0.05 M RbPF₆, 0.5 M CsPF₆, or 0.15 M KPF₆ were compared to acontrol electrolyte with no additives. CsPF₆, RbPF₆, and Sr(PF₆)₂ weresynthesized by mixing stoichiometric amount of AgPF₆ and the iodidesalts of Cs, Rb, or Sr in a PC solution inside a glove box filled withpurified argon where the oxygen and moisture content was less than 1ppm. The formed AgI was filtered out from the solution using 0.45 μmsyringe filters. The electrolyte preparation and lithium deposition wereconducted inside the glove box as well. Lithium films were deposited oncopper (Cu) foil substrates (10 mm×10 mm) in different electrolytesolutions at the desired current densities using a SOLARTRON®electrochemical Interface. After deposition, the electrode was washedwith DMC to remove the residual electrolyte solvent and salt before theanalyses.

Referring to the scanning electron microscope (SEM) micrograph in FIG.2A, when using the control electrolyte, the electrodeposited filmexhibited conventional roughness and dendrite growth. The lithium filmdeposited in the electrolyte with 0.05 M Rb⁺ as the C2 cation exhibits avery fine surface morphology without dendrite formation as shown in FIG.2B. Similarly, for the lithium films deposited with 0.05 M Cs⁺ additive,a dramatic change of the lithium morphology with no dendrite formation(see FIG. 2C) was obtained compared with the control experiment.Surprisingly, although E_(Red)(K⁺) at 0.15 M is theoretically ˜0.06 Vhigher than that of Li⁺ assuming both K⁺ and Li⁺ have an activitycoefficient of 1, K metal did not deposit at the lithium depositionpotential, and a lithium film with a mirror-like morphology was obtainedusing K⁺ as in the additive (FIG. 2D). This experimental findingsuggests that the activity coefficient γ_(x) for K⁺ ion's in thiselectrolyte is much less than those of Li⁺ leading to an actual E_(Red)(K⁺) lower than E_(Red) (Li⁺).

Generally, the concentration of the surface-smoothing additive ispreferably high enough that protrusions can be effectivelyelectrostatically shielded considering the effective ERP, the number ofavailable C2 cations, and the mobility of the C2 cations. For example,in one embodiment, wherein the C2 cation comprises K⁺, the reactantcomprises Li⁺ and C1 comprises lithium metal, the concentration of K⁺ isgreater than 0.05M.

Referring to FIG. 3A, a dendritic lithium film was intentionallydeposited on a copper substrate in a control electrolyte for 1 hour. Thesubstrate and film was then transferred into an electrolyte comprising asurface-smoothing additive, 0.05 M CsPF₆ in 1 M LiPF₆/PC, to continuedeposition for another 14 hours. Unlike the dendritic and mossy filmdeposited in the control electrolyte, the micrograph in FIG. 3B showsthat a smooth lithium film was obtained after additionalelectrodeposition using embodiments of the present invention. Theroughness, pits, and valleys shown in FIG. 3A have been filled by denselithium deposits. The original needle-like dendritic whiskers have beenconverted to much smaller spherical particles which will also be buriedif more lithium is deposited.

FIG. 4 includes SEM micrographs comparing the morphologies of thelithium electrodes after repeated deposition/stripping cycles (2^(nd),3^(rd) and 10^(th) cycle) in cells using the control electrolyte (seeFIGS. 4A, 4B, and 4C) and using electrolyte with a surface-smoothingadditive comprising 0.05M Cs⁺ (see FIGS. 4D, 4E, and 4F). The largelithium dendrites and dark lithium particles are clearly observed on thelithium films deposited in the control electrolyte. In contrast, themorphologies of the lithium films deposited in the Cs⁺-containingelectrolyte still retain their dendrite free morphologies after repeatedcycles. In all the films deposited with the additives, lithium filmsexhibit small spherical particles and smoother surfaces. This is instrong contrast with the needle-like dendrites grown in the controlelectrolyte.

Electrolytes and methods described herein were also applied inrechargeable lithium metal batteries. Coin cells with Li|Li₄Ti₅O₁₂electrodes were assembled using the control electrolyte. Similar cellswere also assembled with electrolytes containing a surface smoothingadditive comprising 0.05 M Cs⁺. FIG. 5 contains SEM micrographs showingthe morphologies of the lithium metal anodes after 100 charge/dischargecycles. Referring to FIG. 5A, the lithium electrode in the cell with noadditive exhibits clear surface roughness and formation of dendrites.However, as shown in FIG. 5B, no dendritic lithium was observed on thelithium electrode in the cell with the surface-smoothing additive, evenafter 100 cycles.

Surface-smoothing additives comprising higher valence cations can alsobe used. Examples include, but are not limited to, Sr²⁺, which haveE_(Red) ^(φ) values of −2.958 V (assuming γ=1) versus a standardhydrogen potential. The lower activity of these cations can result in aneffective ERP lower than that of Li⁺ ions. The larger size and highercharge should be accounted for in the non-aqueous electrolyte. Lithiumfilms were deposited using the control electrolyte along withelectrolytes comprising 0.05 M Sr(PF₆)₂. Deposition from the electrolytecomprising 0.05 M Sr²⁺ results in a lithium film that is smooth, free ofdendrites, and void of Sr in/on the anode. This again indicates that theactivity coefficient for Sr²⁺ in these solutions is less than unity.

Using this approach, C2 cations of the surface-smoothing additive arenot reduced and deposited on the substrate. The C2 cations are notconsumed because these cations exhibit an effective reduction potentiallower than that of the reactant. In contrast, traditionalelectrodeposition can utilize additives having a reduction potentialhigher than that of the reactants; therefore, they will be reducedduring the deposition process and “sacrificed or consumed,” for example,as part of an SEI film or as an alloy to suppress dendrite growth. As aresult, the additive concentration in the electrolyte will decrease withincreasing charge/discharge cycles and the effect of the additives willquickly degrade. In contrast, the C2 cations described herein will forma temporary electrostatic shield or “cloud” around the dendritic tipsthat retards further deposition of C1 in this region. This “cloud” willform whenever a protrusion is initiated, but it will dissipate onceapplied voltage is removed or the protrusion is eliminated. Accordingly,in some embodiments, the applied electrical potential is of a value thatis less than, or equal to, the ERP of the reactants and greater than theeffective ERP of the cations of C2.

Lithium films having an SEI layer on the surface and deposited usingelectrolytes comprising 0.05 M Cs⁺, Rb⁺, K⁺, or Sr²⁺ additives wereanalyzed by x-ray photoelectron spectroscopy (XPS), Energy-dispersiveX-ray spectroscopy (EDX) dot mapping, and Inductively coupled plasmaatomic emission spectroscopy (ICP/AES) methods. XPS and EDX results didnot show Cs, Rb, K, and Sr elements in the SEI films within thedetectable limits of the analysis instruments. In addition, ICP-AESanalysis did not identify Cs, Rb, K, and Sr elements in the bulk ofdeposited lithium film (including the SEI layer on the surface) withindetectable limits.

Dendrite formation is not only a critical issue in rechargeable lithiummetal batteries, but also an important issue in high power lithium ionbatteries because lithium metal dendrites can grow at the anode surfacewhen the lithium ions cannot move quickly enough to intercalate into theanode, which can comprise graphite or hard carbon, during rapidcharging. In this case, the lithium dendrites can lead to short circuitsand thermal runaway of the battery. Accordingly, a carbonaceous anode isdescribed herein to demonstrate suppression of lithium dendrite growthin a lithium ion battery. FIG. 6 compares the optical (6A and 6D) andSEM images (6B, 6C, 6E, and 6F) of lithium particles formed on the hardcarbon anode after it was charged to 300% of its theoretical capacity ina control electrolyte (without additives) and in an electrolyte having asurface smoothing additive comprising 0.05 M CsPF₆. A significant amountof lithium metal was deposited on the surface of carbon electrode (seegrey spots in FIG. 6A) for the sample overcharged in the controlelectrolyte. FIGS. 6B and 6C show clear dendritic growth on theelectrode surface. In contrast, no lithium metal deposition was observedon the surface of carbon electrode (see FIG. 6D) for the sampleovercharged in the electrolyte with 0.05M Cs⁺ additive (the white lineon the bottom of the carbon sample is due to an optical reflection).After removing a small piece of carbon from the sample (see the circledarea in FIG. 6D), it was found that excess lithium was preferentiallygrown on the bottom of the carbon electrode as shown in FIGS. 6E and 6F.

While a number of embodiments of the present invention have been shownand described, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims, therefore, areintended to cover all such changes and modifications as they fall withinthe true spirit and scope of the invention.

We claim:
 1. A method for improving surface smoothness duringelectrodeposition of a first conductive material (C1) on a substratesurface, the method comprising: providing an electrolyte solutioncomprising reactants from which C1 is synthesized and a soluble,surface-smoothing additive comprising cations of a second conductivematerial (C2), wherein cations of C2 have a) a standard electricalreduction potential that is greater than an electrochemical reductionpotential of the reactants and (b) an activity in solution such that aneffective electrochemical reduction potential of the cations of C2 inthe solution is lower than the electrochemical reduction potential ofthe reactants; and applying an electrical potential that is less thanthe electrochemical reduction potential of the reactants and greaterthan the effective electrochemical reduction potential of the cations ofC2, thereby reducing the reactants and forming C1 on the substratesurface.
 2. The method of claim 1, further comprising: accumulatingcations of C2 at protrusions on the substrate surface, thereby formingan electrostatically shielded region near each protrusion; andtemporarily repelling reactants from the electrostatically shieldedregion near each protrusion.
 3. The method of claim 1, wherein C1 is ametallic material and the reactants comprise cations of C1.
 4. Themethod of claim 3, wherein C1 is selected from a group consisting of Na,K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Al, Ga, In, Tl, Ge, Sn, Pb, As, Sb, Bi,Se, Te, Bi, Po, Sc, Tl, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo,Tc, Ru, Rh, Pd, Ag, Cd, W, Pt, Au, Hg, and combinations thereof.
 5. Themethod of claim 3, wherein C1 comprises Li.
 6. The method of claim 1,wherein C1 comprises an electronic conductive polymer and the reactantscomprise monomers of the polymer.
 7. The method of claim 1, wherein thecations of C2 are metal cations.
 8. The method of claim 7, wherein thecations of C2 comprise a metal selected from a group consisting of Li,Cs, Rb, K, Ba, La, Sr, Ca, Ra, Zr, Te, B, Bi, Ta, Ga, Eu, S, Se, Nb, Na,Mg, Cu, Al, Fe, Zn, Ni, Ti, Sn, Sb, Mn, V, Ta, Cr, Au, Ge, Co, As, Ag,Mo, Si, W, Ru, I, Fc, Br, Re, Bi, Pt, Pd, and combinations thereof. 9.The method of claim 1, wherein the cations of C2 have a concentration inthe electrolyte solution that is less than 10% of that of the reactants.10. The method of claim 1, wherein the concentration of the cations ofC2 have a concentration in the electrolyte solution that is lees than,or equal to, 5% of that of the reactants.
 11. The method of claim 1,wherein the surface-smoothing additive comprises an anion selected froma group consisting of PF₆ ⁻, AsF₆ ⁻, BF₄ ⁻, N(SO₂CF₃)₂ ⁻, N(SO₂F)₂ ⁻,CF₃SO₃ ⁻, ClO₄ ⁻, I⁻, Cl⁻, OH⁻, NO₃ ⁻, SO₄ ²⁻, and combinations thereof.12. The method of claim 1, wherein the substrate is an electrode. 13.The method of claim 12, wherein the electrode comprises lithium.
 14. Themethod of claim 12, wherein the electrode comprises carbon.
 15. A methodfor improving surface smoothness during electrodeposition of lithium ona substrate surface, the method comprising: providing an electrolytesolution comprising lithium cations and a soluble, surface-smoothingadditive comprising cations of a second conductive material (C2)selected from a group consisting of cesium, rubidium, potassium,strontium, barium, calcium, and combinations thereof, wherein cations ofC2 have an activity in solution such that an effective electrochemicalreduction potential of the cations of C2 is lower than anelectrochemical reduction potential of the lithium cations; applying anelectrical potential that is less than the electrochemical reductionpotential of the lithium cations and greater than the effectiveelectrochemical reduction potential of the cations of C2, therebyreducing the lithium cations and forming lithium on the substratesurface; accumulation cations of C2 at protrusions on the substratesurface, thereby forming an electrostatically shielded region near eachprotrusion; and temporarily repelling the lithium cations from theelectrostatically shielded region near each protrusion.
 16. The methodof claim 15, wherein the cations of C2 have a concentration in theelectrolyte solution that is less than 10% of that of the lithiumcations.
 17. The method of claim 15, wherein the cations of C2 have aconcentration in the electrolyte solution that is less than, or equalto, 5% of that of the lithium cations.
 18. The method of claim 15,wherein the surface-smoothing additive comprises an anion comprising PF₆⁻ anion.
 19. The method of claim 15, wherein the substrate is a batteryanode comprising lithium or a battery anode comprising carbon.